A polyester-based polymer such as a bimolecularly cyclized ester, e.g., dilactide and glycolide, a monomolecularly cyclized ester, e.g., lactone, and a copolymer thereof (hereinafter, referred to as "polyester-based polymer") are decomposed by light, heat, oxygen, and the like, and those are entered into a natural recirculation system. Therefore, many investigations have been recently conducted for the uses as a biodegradable polymeric material from the viewpoint of safeness and prevention of global environmental pollution.
As a process for the preparation of a homopolymer of dilactide or diglycolide which is a bimolecularly cyclized ester, there have been conventionally known preparation processes which are divided into two groups.
That is, one of the processes is a method for directly obtaining a polymer by dehydropolycondensation of the corresponding hydroxycarboxylic acids. The other is a method for obtaining a polymer by synthesis of a dehydrated cyclic ester of a hydroxy acid known as examples of dilactide and diglycolide, and then by the ring opening polymerization of the ester to obtain a polymer.
According to the former direct polycondensation method, it is difficult to obtain a polymer having a molecular weight of not less than 4,000 ("Lactic acid" in Veriag Chemie, page 226, 1971, which is written by C. H. Halten) and, even though an attempt is conducted for increasing the molecular weight by studies of the reaction conditions, the molecular weight is limited in approximately 20,000 at largest as shown in JP-B-02052930. In the case that there is required the preparation of a polymer having a higher molecular weight, there has been conventionally employed the latter method which is the ring opening polymerization of a cyclic esterified product.
Further, as a continuous process for the preparation of the lactides or lactones, a continuous process for the preparation of aromatic polyester and lactones is disclosed as "a continuous process for the preparation of an elastic polyester" in JP-A-61261124, JP-A-61283619, JP-A-61287922, JP-A-62020525, and JP-A-62027425 Official Gazettes and "a process for the preparation of an elastic polyester" in JP-A-02302433 and JP-A-02302434 Official Gazettes. In all the processes, there are equipped screw-type or paddle-type agitating blades such as a kneader or an extruder in an inside of a reaction vessel, and a reaction system is agitated by the agitating blades while the reactants are sequentially moved from an inlet for feeding raw materials to an outlet for a product.
Still further, as a continuous process for the preparation of the lactides, JP-A-05093050 Official Gazette discloses a so-called CSTR continuous preparation process in which a plurality of agitating vessels are connected in series each other, and raw materials for the reaction are continuously supplied, whereby, a continuous polymerization is conducted within a retention time of period which is a reaction period between initial reaction vessel and final reaction vessel.
However, in all the processes, there is employed a reaction apparatus equipped with a dynamic agitator, and the processes do not give a disclosure and suggestion concerning a solution to difficulty in uniform agitation and difficulty in heat removal caused by an elevation in the viscosity of the reactants through the continuous preparation of a biodegradable polyester-based polymer having a higher molecular weight from lactides or lactones.
That is, even though there are tried the continuous processes for the preparation of the lactides in the JPs or literature as described hereinabove, viscosity in the polymer elevates to a very high viscous region such as 10,000-several hundred of thousands poise with an increase of an average molecular weight in the polymer produced, resulting in that agitation by a usual agitator becomes difficult, and it becomes difficult to take out reactants. Also, even though a powerful agitator is employed, and a reaction system is agitated by employing agitation blades devised, the reactants move only in a substantially laminar flow according to the rotation of the agitation blades, resulting in that it becomes difficult to uniformly mix an inside of a reaction system as a whole.
In addition, since the ring-opening polymerization of cyclic esters exothermally proceeds, it becomes difficult to control a temperature in a reaction vessel by difficulty in uniform agitation because of an increase in viscosity, whereby, the reaction occasionally proceeds in an abnormal state, and temperature distribution is also caused in the reaction system, resulting in that deterioration in quality of a polymer is caused by being locally heated.
In recent years, in order to solve the problems, there has been employed (for example, U.S. Pat. No. 5,484,882) a static mixer (hereinafter, referred to as SMX) which does not have a movable part, that is, an agitator. However, flow resistance becomes very large because of a structure by which flow is repeatedly divided, converted, and reversed by unmovable mixing elements.
In other words, pressure loss becomes very large in a reaction system, and it becomes difficult to design a reaction vessel and pumps, etc. Moreover, decline in productivity is caused by an upper limit in discharging pressure.
In the case that the SMX is employed, there is possible only a most suitable design for a specified operation condition because of the absence of a movable part by which mixing, that is, shear rate is controlled, and mixing cannot be controlled in other operation conditions, that is, almost of operation conditions, resulting in that an operation cannot help being conducted while including inferiority in mixing and thermal distribution which always exceed a range. Further, in order to lower a higher pressure loss, in the case that there increases a diameter of the SMX, that is, area of a section through which a fluid passes, inferiority in mixing and thermal distribution become exceedingly large. Although the SMX is occasionally employed as a continuous reaction apparatus which is a recirculation style in order to avoid inferiority in mixing and thermal distribution, and a mixing effect is improved by repeatedly recirculating, there is enlarged a distribution of retention period in an inside of a reaction vessel, and there cannot be avoided deterioration of quality such as decomposition and discoloration in a polymer by a long time heating. Still further, because flow volume increases in a recirculating line, a plant becomes gigantic, plant costs become very expensive, and it is not practical.
Also, there is disclosed a process (eg. U.S. Pat. Nos. 5,378,801 and 5,468,837) in which an extruder is employed for polymerization of a polyester-based polymer. In the process, in the case that polymerization is conducted in an industrial scale, there is a problem that it is difficult to remove heat, resulting in that quality in a polymer becomes occasionally worse.
Particularly, the polyester-based polymer prepared from a cyclic ester has an excellent biodegradability. However, it is readily hydrolyzed by acids, alkalis, or water, and there is apt to be caused a decline in a molecular weight also by heating. For example, in GUPTA M. C., "Colloid Polymer Science", (Deu) 260 (3) 308-311, 1982, there is reported an investigation example of thermal decomposition rate in a homopolymer of dilactide by a thermogravimetric analysis. However, there is caused an accelerated decline in a molecular weight at high temperatures such as not less than 250.degree. C. even in a hermetically sealed reaction vessel.
In addition, the homopolymer or copolymer of dilactide has a property that discoloration proceeds by being exposed to high temperature. That is, in the conventional process for the continuous polymerization of polymers from cyclic ester monomers, uniform mixing is prevented because of an increase of viscosity in the polymers caused by an increase of molecular weight, resulting in causing a problem that denaturation is partially caused by being locally heating, and quality is lowered.
Accordingly, setting aside a small scale experiment, there has been desired a more preferred process for an industrial mass production. It is a purpose of the present invention to provide a continuous process for the preparation of a polyester-based polymer having an excellent quality, by which there can be solved a difficulty in uniformly mixing caused by an increase of viscosity in reactants, a difficulty in removal of heat, and a decline in productivity by a high pressure loss, and which would become problematic in the case of industrially preparing the polyester-based polymer.